This invention relates to scintillation camera systems and more specifically to scintillation camera systems having high speed pulse shaping circuitry for minimizing base line fluctuation and voltage undershooting.
In the diagnosis of certain illnesses, radioactive isotopes are administered to the patients. These isotopes have the characteristic of concentrating in certain types of tissue. The degree of concentration in the tissue is dependent upon tissue type. For example, iodine 131 generally collects or concentrates in the tissue of the thyroid gland. Upon detection of the level or radioactive isotope concentration and presentation of this detected information on a suitable readout device, such as an oscilloscope, it is frequently possible to diagnose the condition of the tissue under examination.
One well known type of device for detecting levels of radioisotope concentration is the scintillation camera system. Scintillation cameras generally incorporate a relatively large disc-shaped scintillation crystal which is positioned so that the crystal intercepts gamma radiation emitted by a patient under study. The crystal scintillates in response to impinging gamma ray energy to provide pulses of light energy. A thallium activated sodium iodide crystal is typically employed as the scintillation crystal.
A plurality of phototubes are positioned adjacent the crystal so that a scintillation occurring in the crystal is normally detected by several of the phototubes. Each of the detecting phototubes develops in response to the scintillation an electrical signal having an amplitude proportional to the intensity of the light energy received by it. The signals developed by the phototubes are then amplified and applied to appropriate electronic computing circuitry for developing electrical signals representative of the position of the light pulse or scintillation. The intensity of the signal is threshold detected by a process commonly referred to as pulse height analysis to determine whether the signals represent photopeak scintillations resulting from a gamma ray originatng from the isotope which has been administered to the patient. The signals developed by the phototubes are typically preamplified to provide signals having relatively narrow pulse widths to enable as high a processing rate as possible. One such gamma ray imaging camera system is disclosed in U.S. Pat. No. 3,683,180 issued Aug. 8, 1972.
At high counting rates this type of camera system has several difficulties which have hindered high speed operation. Coupling capacitors used for transmission of the data pulses are intermittently charged and discharged by the pulse bursts passing through the capacitors. This results in considerable base line bounding and lessens the fidelity of data detection. Because these pulses are pulse height analyzed by threshold detectors, base line bouncing tends to introduce pulse height detection errors.
Scintillation camera systems now commonly utilize delay line clipping techniques for narrowing the pulse width of the data pulses for obtaining higher counting rates with minimum distortion of the pulse amplitudes.
Known delay line clipping circuits utilize a delay line to couple the pulse transmission line to circuit ground. One input of a differential amplifier is coupled to the pulse transmission line and to the delay line while the other input of the differential amplifier is coupled to circuit ground. When using an ideal delay line, ideal resistors, and ideal amplifiers, a step input pulse is shortened to a square pulse of width twice the delay time of the delay line. Since delay lines have unavoidable attentuation and are not ideal, the base lines of the clipped pulses do not return to the original value. After the clipping they exhibit an offset proportional to the original step input voltage and to the delay line attenuation. Furthermore, since the input pulse is finite with a decaying trailing edge rather than a step, the resultant clipped pulse frequently undershoots the base line voltage.
Attempts have been made in systems other than scintillation camera systems to remove the base line fluctuations of single delay line clipped pulses. One such base line restoring circuit is that often referred to as an amplified diode restorer. Due to the finite forward breakover voltage of a diode restorer, even upon amplification, the restorer is unable to work properly below a threshold voltage of approximately 30 millivolts. This level of threshold voltage is unacceptable in radiation imaging systems as accurate pulse intensity data is required down to substantially zero volts. The accurate detection of such small voltages is required if the situs of respective scintillations is to be accurately reconstructed.
Double delay line clipping networks have been utilized to narrow pulses which have relatively long exponential fall times in nuclear spectroscopy systems. It is known that such double delay line clipped pulses are advantageous in certain respects over comparable single delay line clipped pulses. That is, the total charge deposited on any coupling capacitor in the date transmission line due to passage of each pulse is zero because the double delay line pulse exhibits equal energies above and below the base line. Accordingly, the average base line value is relatively constant; whereas, the single delay line clipped pulse exhibits a base line which is astable at high rates due to random time variations in the average charge passing through coupling capacitors.
Double delay line clipped pulses have their disadvantages when used as data pulses in a scintillation camera system. For example, for a given pulse duration the effective integration time of the photomultiplier tube generating the pulse is reduced. Conversely, if the integration time is kept equal to that of a single delay line clipped pulse, the total pulse duration is lengthened, resulting in a slower operating system.
On the other hand, double delay line data pulses are ideal timing pulses. The pulse widths may be sufficiently narrow to accommodate high frequency data pulses separated by as little as 1.5 microseconds. Because the base line of a double delay line pulse is stable, restoring circuitry is unneeded.
The present invention overcomes the above noted and other disadvatages by providing a novel base line restoring circuit and a novel delay line clipping circuit in a scintillation camera system. Single and double delay line clipped signal waveforms are generated for increasing the operational frequency and fidelity of data detection of the camera system which is otherwise degraded by base line distortion such as undershooting, overshooting, and capacitive build-up.
The camera system includes a set of photomultiplier tubes and associate amplifiers which generate sequences of pulses. These pulses are pulse-height analyzed for detecting a scintillation having an energy level which falls within a predetermined energy range. Data pulses which have the predetermined energy are said to represent photopeak events of the isotope which has been administered to the patient. These data pulses are combined to provide x+, x-, y+,y-coordinate data representative of the situs of a photopeak event and to provide Z energy data representative of the energy of the photopeak event. The pulses are characteristically produced having a relatively long decaying trailing edge by dynamic biasing of the preamplifier. The preamplifiers are biased out of saturation over all ranges of pulse energy levels and count rates.
Single delay line clipping circuitry is provided for narrowing the pulse width of the decaying electrical data pulses which increase operating speed the occurrence of data loss. According to one aspect of the invention, the clipping circuitry is compensated and includes a variable resistance element which controls magnitude and polarity of portions of an unclipped data pulse and adds these portions to a delay line clipped data pulse for substantially eliminating undershooting base line voltage.
In another embodiment, a novel base line restorer circuit is provided which is advantageously used prior to any thresholding to the data pulses to allow precise amplitude analysis. The restorer circuit requires timing pulses which are derived from double delay line type pulses, because of their inherently stable base line. The single delay line clipping circuitry is combined in a novel circuit for providing double delay line type pulses without the requirement of a second delay line. The delay line of the single delay line clipping circuitry for the energy channel is simultaneously used for generating the double delay line pulses. The double delay line pulses are substantially synchronized in time with the single delay line clipped data pulses and are used as timing pulses.
The base line restorer circuit includes a gating element and a coupling capacitor connected in series with a data line over which the data pulses are transmitted. The gating element is responsive to the double line clipped timing pulse for selectively coupling the data line to a reference potential such as circuit ground. This discharges the coupling capacitor in anticipation of a data pulse and thereby provides the single delay line shaped pulses with improved base line voltage characteristics.
It accordingly is a general object of this invention to provide a scintillation camera having novel and improved pulse-shaping clipping circuitry for maximizing system speed without incurring data loss.
Other objects and advantages and a fuller understanding of the invention may be obtained by referring to the following detailed description when read in conjunction with the accompanying drawings.